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Mercury increase in Pacific yellowfin tuna

Mercury concentrations in Hawaiian yellowfin tuna populations (Thunnus albacares) are increasing at a rate of 3.8 percent or more per year, according to a new University of Michigan-led study. Mercury is a toxic, bioaccumulating trace metal whose emissions to the environment have increased significantly as a result of anthropogenic activities such as mining and fossil fuel combustion. With the great deal of mercury actively cycling in the environment at present, one might predict that the concentration of mercury in fish should have increased dramatically since the Industrial Revolution. For decades scientists have expected these mercury levels in open-ocean fish to increase in response to rising atmospheric concentrations, however evidence for that hypothesis has been hard to find.

This recent study led by Paul Drevnick, an assistant research scientist at the U-M School of Natural Resources and Environment compiled and re-analysed three previously published reports on yellowfin tuna caught near Hawaii. These three studies sampled the same yellowfin tuna population near Hawaii in 1971, 1998 and 2008. In each of the three studies, muscle tissues were tested for total mercury, nearly all of which was the toxic organic form, methylmercury. In their re-analysis, Drevnick and his colleagues included yellowfins between 48 and 167 pounds and used a computer model that controls for the effect of fish body size. Data from 229 fish were analysed: 111 from 1971, 104 from 1998 and 14 from 2008. The researchers found that mercury concentrations in the yellowfins did not change between the 1971 and 1998 datasets. However, concentrations were higher in 2008 than in either 1971 or 1998. Between 1998 and 2008, the mercury concentration in yellowfins increased at a rate greater than or equal to 3.8 percent a year, according to the new study. In a U-M press release, Drevnick says:

“The take-home message is that mercury in tuna appears to be increasing in lockstep with data and model predictions for mercury concentrations in water in the North Pacific. This study confirms that mercury levels in open ocean fish are responsive to mercury emissions.”

“Mercury levels are increasing globally in ocean water, and our study is the first to show a consequent increase in mercury in an open-water fish. More stringent policies are needed to reduce releases of mercury into the atmosphere. If current deposition rates are maintained, North Pacific waters will double in mercury by 2050.”

One of the major concerns regarding the accumulation of mercury in fish like Yellowfin tuna is that such species are commonly fished and consumed by humans, thus exposing humans to elevated levels of methylmercury.Yellowfin tuna, often marketed as ahi, is widely used in raw fish dishes—especially sashimi—or for grilling. The Natural Resources Defence Council’s guide to mercury in sushi lists yellowfin tuna as a “high mercury” species. It is estimated 40% of the mercury that eventually finds its way into fish originates with coal-burning power plants and chlorine production plants. This atmospheric mercury can make its way into the food chain via rain or snow, which may flow into bodies of water like lakes and streams. When it falls out of the air as dry deposition, it may eventually be washed into those bodies by rain. Bacteria in soils and sediments convert mercury to methylmercury and in this form, it is taken up by tiny aquatic plants and animals. Consumption of these organisms by fish build up methylmercury levels in their bodies and as increasingly larger fish species consume smaller ones, the methylmercury is concentrated further up the food chain. This process is called bioaccumulation1 and can potentially result in humans being exposed to high levels of methylmercury.

In the United States, the EPA serves as an advisory organ to set the levels of mercury that are non-fatal in humans. Exposure to high levels of methyl mercury include disturbed vision, hearing, and speech, lack of coordination, and muscle weakness. In fetuses, infants, and children, the primary health effect of methylmercury is impaired neurological development. Thus it is important for pregnant women to avoid situations where they may be exposed to excess methylmecury. Methylmercury exposure in the womb, which can result from a mother’s consumption of fish and shellfish that contain methylmercury, can adversely affect a baby’s growing brain and nervous system. Impacts on cognitive thinking, memory, attention, language, and fine motor and visual spatial skills have been seen in children exposed to methylmercury in the womb1.

Recent emissions estimates of annual global mercury emissions from all sources, natural and anthropogenic (human-generated), which are highly uncertain, are about 5,000-8,000 metric tons per year. U.S. anthropogenic mercury emissions are estimated to account for roughly three percent of the total global emissions, and the U.S. power sector is estimated to account for about one percent the total global emissions2.

In general, eating fish can be very healthy and an important source of nutrition for us humans. Thus it is important that we limit the amount of fish we eat that contain high levels of methylmercury. The National Resources Defence Council has compiled this guide for those wishing to control their methylmercury intake, using data from the Food and Drug Administration and the Environmental Protection Agency:

*Fish in troubleThese fish are perilously low in numbers or are caught using environmentally destructive methods. To learn more, see the Monterey Bay Aquariumand the The Safina Center (formerly Blue Ocean Institute), both of which provide guides to fish to enjoy or avoid on the basis of environmental factors.